The photographic arts have a long-standing history of attempts to supply customers with convenient and simple ways to take pictures that serve to record their everyday lives. Conventional color photography has attempted to meet this need by supplying light sensitive silver halide containing films suitable for use in hand-held cameras. Upon exposure, the film carries a latent image that is only revealed after suitable processing. These elements have historically been processed by treating the camera-exposed film with at least a developing solution having a developing agent that acts to form image.
The well known chromogenic dye-forming films require reducing agents such as p-aminophenols or p-phenylenediamine developers to form dye images. These reducing agents are typically present in developer solutions which are then brought into reactive association with exposed photographic film elements at the time of processing. Segregation of the developer and the film element has been necessary because the incorporation of developers directly into sensitized photographic elements frequently leads to desensitization of the silver halide emulsion and undesirable fog. Considerable effort has therefore been directed at trying to produce effective blocked developers, which can be introduced in silver halide emulsion elements without deleterious desensitization or fog effects and which un-block under conditions of development so that developer is free to participate in image-forming (dye or silver metal forming) reactions.
U.S. Pat. No. 3,342,599, to Reeves, discloses the use of Schiff base developer precursors. Schleigh and Faul, in a Research Disclosure (129 (1975) pp. 27–30), described the quaternary blocking of color developer and the acetamido blocking of p-phenylenediamines. (All Research Disclosures referenced herein are published by Kenneth Mason Publications, Ltd., Dudley Annex, 12a North Street, Emsworth, Hampshire P010 7DQ, ENGLAND.) Subsequently, U.S. Pat. No. 4,157,915, to Hamaoka et al., and U.S. Pat. No. 4,060,418, to Waxman and Mourning, describe the preparation and use of blocked p-phenylenediamines in an image receiving sheet for color diffusion transfer.
All of these approaches have failed in practical product applications because of one or more of the following problems: desensitization of sensitized silver halide; unacceptably slow unblocking kinetics; instability of blocked developer yielding increased fog and/or decreased Dmax after storage, and lack of simple methods of releasing the blocked developer.
Recent developments in blocking and switching chemistry have led to blocked p-phenylenediamines that perform well. In particular, compounds having “β-ketoester” type blocking groups (strictly, β-ketoacyl blocking groups) are described in U.S. Pat. No. 5,019,492. With the advent of the β-ketoester blocking chemistry, it has become possible to incorporate p-phenylenediamine developers in film systems in a form from which they only become active when required for development.
The β-ketoacyl blocked developers are released from the film layers in which they are incorporated by an alkaline developing solution containing a dinucleophile, for example hydroxylamine.
The incorporation of these blocked developers in photographic elements is typically carried out using colloidal gelatin dispersions of the blocked developers. These dispersions are prepared using means well known in the art, wherein the developer precursor is dissolved in a high vapor pressure organic solvent (for example, ethyl acetate), along with, in some cases, a low vapor pressure organic solvent (such as dibutylphthalate), and then emulsified with an aqueous surfactant and gelatin solution. After emulsification, usually done with a colloid mill, the high vapor pressure organic solvent is removed by evaporation or by washing, as is well known in the art.